Generally, polyamides for molding materials have been molded into shaped articles by injection molding or the like. Therefore, polyamides have been required to have a high fluidity upon melting, i.e., low viscosity polyamides have been used as the molding materials. However, when applied to production of bottles, sheets, films, fibers or the like, polyamides are molded into shaped by extrusion in addition to injection molding. Therefore, the polyamides for use in these applications are required to have a lower fluidity than those of the polyamide for use as the molding materials, and medium- to high-viscosity polyamides have been used.
As low-viscosity polyamides to be used mainly as the molding materials, polyamides obtained by melt-polycondensation have been used directly or after drying. However, when it is intended to obtain medium- to high-viscosity polyamides applicable to production of bottles, sheets, films, fibers or the like by the melt-polycondensation, a special polymerization reactor is needed to keep the contents in a polymerization reactor in uniform molten state, because ordinary agitators cannot produce agitating force sufficient for maintaining the uniform molten state. Further, when the polycondensation is continued until a low-viscosity polyamide is converted to a medium- to high-viscosity polyamide, the time required for maintaining the molten state (reaction time) is considerably prolonged. As a result, there arises damage of polyamide molecules (deteriorated polymer molecules due to generation of radicals), or occurrence of aberrant reactions such as non-linear molecular growth (production of three-dimensional polymers), thereby increasing the amount of gels and fish eyes which causes disadvantages in practical use. If such polyamides containing a large amount of gels and fish eyes are used for production of bottles, sheets, films, fibers or the like, defective products occur with extremely high frequency, resulting in deteriorated productivity. Even though a filter is used upon molding, it is difficult to completely remove gels and fish eyes from polyamides. Further, the filter must be replaced with new ones more frequently, this reducing the continuous production run. Therefore, it is desirable that the amount of gels and fish eyes in polyamides is as small as possible.
It is known that medium- to high-viscosity polyamide containing gels or fish eyes in small amount can be obtained by producing low-viscosity polyamide by melt-polycondensation, and then heat-treating the resultant low-viscosity polyamide in solid phase (so-called solid phase polymerization). The difference in the amounts of gels or fish eyes between melt-polycondensation and solid phase polymerization is attributable to the difference in the frequency of occurrence of damages to polyamide molecules and aberrant reactions due to different reaction temperatures. Thus, medium- to high-viscosity polyamide obtained by solid phase polymerization contain gels or fish eyes in smaller amount as compared with those obtained only the melt-polycondensation. However, in the production of bottles, sheets, films, fibers or the like, the productivity of these products is considerably affected even when gels or fish eyes are present in slight amounts. Therefore, it has been demanded to further reduce the amounts of gels or fish eyes in solid phase-polymerized polyamides.
Gels or fish eyes are formed not only during the production of polyamides but also during the melting for molding polyamides into shaped articles. Even though polyamides show no considerable difference in the amounts of gels or fish eyes, molded articles contain, in some cases, different amounts of gels or fish eyes. One reason therefor may be that slight differences in damages to polyamide molecules and slight differences in the occurrence of aberrant reactions, which are not detected just after the production of polyamides, are increased by stagnation of polyamides in filter, molding die, etc. during the molding process. Thus, to obtain molded articles containing gels or fish eyes in small amounts, it is necessary to design a special molding apparatus having few stagnating portions where polyamides are retained not flowing. Simultaneously, it is essentially required to produce high-quality polyamides by preventing damages to molecules and aberrant reactions in melt-polymerization and solid phase polymerization.
Amorphous polyamide granules, i.e., granules of poly-m-xylylene adipamide having a crystallinity of not more than 13% transfer from amorphous state to crystalline state when heated to a temperature higher than a glass transition temperature. The amorphous polyamide granules abruptly become tacky when heated to near the glass transition temperature, and remain tackiness until the polyamide is crystallized. Solid phase polymerization is effected by the heat transferred from a heating medium kept at a temperature higher than that of the polyamide. When polyamide granules fail to move freely and stagnate in the vicinity of heat transfer surface of inner wall of a heating apparatus, the polyamide granules tackifiedly stick to the wall surface of the heating apparatus. Also, the polyamide granules tackifiedly stick to each other to form massive granules. When the tackifiedly stuck granules are crystallized without disintegration, there arises disadvantage of solidified sticking of the granules. If solid phase polymerization is continued after crystallization without disintegration of the solidified sticky massive granules, solid phase-polymerized polyamides having a uniform degree of polymerization cannot be obtained. In addition, damages to polyamide molecules and aberrant reactions are likely to occur due to partial heating, thereby inducing the formation of gels or fish eyes.
To avoid the above disadvantages, there have been generally employed the following processes in subjecting amorphous polymers to solid phase polymerization:
(a) Batchwise process where polymer is heated gently in a batchwise heating apparatus such as a rotary drum in an inert gas atmosphere or under reduced pressure, thereby crystallizing the polymer while avoiding the tackified sticking of the polymer granules, and then further heating the polymer in the same heating apparatus to carry out solid phase polymerization; PA0 (b) Continuous process where the polymer is heated in a channel stirring heating apparatus in an inert gas stream to crystallize the polymer (pre-crystallization), and then the crystallized polymer is subjected to solid phase polymerization in a hopper heating apparatus in an inert gas stream. PA0 (c) Semi-continuous process where the polymer is crystallized in a channel stirring heating apparatus, and then the crystallized polymer is subjected to solid phase polymerization in a batchwise heating apparatus such as a rotary drum.
The following problems arise when solid phase polymerization of an amorphous polyamide constituted by a diamine component composed mainly of m-xylylenediamine and a dicarboxylic acid component composed mainly of adipic acid is carried out by following the above conventional processes.
In a batchwise heating apparatus such as rotary drum as used in the above process (a), agitation and mixing sufficient for disintegrating massive granules formed by tackified sticking and solidified sticking of polyamide granules cannot be obtained, so that the rotation of the rotary drum is disturbed and problems of decentering and power fluctuation occur. Therefore, it has been attempted to employ operational conditions capable of avoiding the tackified sticking of polyamide granules. Specifically, the temperature of heating medium is kept low so as to reduce the temperature rise rate of polyamide granules until the crystallization is completed, or polyamide granules are vigorously moved by reducing a filling rate and increasing a rotational speed of the rotary drum. However, it has been very difficult to avoid the tackified sticking of polymer granules over a period after reaching the glass transition temperature and until completing the crystallization. Therefore, the amount of polymer granules to be charged into the drum must be reduced to a level causing no mechanical trouble even though the tackified sticking of polymer granules occur. Thus, the decrease in productivity is unavoidable in the conventional processes.
Although a channel stirring heating apparatus as used in the processes (b) and (c) is effective for mechanically disintegrating massive granules of takifiedly sticky polyamide and solidifiedly sticky polyamide, it is still required that the temperature of heating medium is kept low until the crystallization of polyamide granules is completed so as to prevent polyamide granules from tackifiedly sticking to inner wall and agitation blades of the heating apparatus. Further, since the channel stirring heating apparatus cannot be sufficiently sealed as compared with a rotary drum, the heating apparatus is not suitable for treating polymers such as polyamide which is susceptible to yellowing due to thermal oxidation, even when polymers are heated in an inert gas stream. Also, the channel stirring heating apparatus requires a large amount of highly-pure inert gas. In addition, a larger amount of powders is generated as compared with using a rotary drum, resulting in contamination of polyamide granules.
Japanese Patent Publication No. 49-28679(1974) discloses a process for producing polyamide 6 and polyamide 12. In the proposed process, a low-viscosity polyamide obtained by polymerization in the presence of specific amounts of an organic acid, as a chain stabilizer, selected from monocarboxylic acids and dicarboxylic acids, and an inorganic or mineral acid as a polymerization catalyst is subjected to solid phase polymerization. The solid phase polymerization is carried out by heating the low-viscosity polyamide for a long period of time until the viscosity of polyamide reaches a certain final value independent of the residence time. In this process, since the inorganic or mineral acid not only acts as a catalyst for amidation reaction but also accelerates aberrant reactions mentioned above, the formation of gels are apparently unavoidable especially in the polyamide of the present invention. In addition, the reaction time of solid-phase polymerization is prolonged merely until the polymer reaches an equilibrium molecular weight at the reaction temperature. Therefore, it is difficult to reduce the amounts of gels or fish eyes in the polyamide of the present invention by the proposed process.
Japanese Patent Publication No. 50-2197(1975) discloses a process for producing polyamide 6, which comprises pre-treatment of polyamide 6 pellets by adding water thereto so as to adjust the water content to not less than 1.0% by weight and then heating the pellets under pressure, and solid phase polymerization of the polyamide pellets after drying, thereby shortening the reaction time of solid phase polymerization. In this process, the pre-treatment is carried out in the presence of water vapor to reduce the reaction time of solid phase polymerization. In the pre-treatment, since no sufficient molecular growth is achieved, it is not considered that damages to polyamide molecules and occurrence of aberrant reactions during the solid phase polymerization can be effectively prevented. Further, the process requires the use of a pressure-type heating apparatus.
Japanese Patent Application Laid-Open No. 7-90076(1995) discloses a process of subjecting polyamide 6 or the like to solid phase polymerization in steam atmosphere to prevent the formation of gelated products. However, there is no description as to essential properties of the melt-polymerized polymer to be subjected to solid phase polymerization.
Japanese Patent Application Laid-Open No. 1-284526(1989) discloses a process for producing ultra-high molecular weight polyamide 66 by subjecting polyamide 66 having a specific end group balance to solid phase polymerization. However, the end group balance ranges from diamine excess to dicarboxylic acid excess. Therefore, the object of the proposed process is to produce polyamide 66 having a number-average molecular weight of 100,000 or higher, and there is no teaching about the reduction of gels and fish eyes.
Japanese Patent Application Laid-Open No. 4-197710(1992) describes a method of crystallizing polyester chips. In this method, synthetic resin chips as a fed material are crystallized in a channel stirring heating apparatus while forming water film on the chips by feeding water or steam to the heating apparatus. As described above, since the channel stirring heating apparatus is used, the obtained products are likely to have deteriorated quality due to yellowing, etc. Thus, the method is unsuitable for the production of the polyamide of the present invention.
In Japanese Patent Application Laid-Open No. 56-149431(1981), proposed is a process for subjecting polytetramethylene adipamide to solid phase polymerization in a steam-containing atmosphere to prevent coloring of products. In this process, a prepolymer containing an excessive amount of 1,4-diaminobutane is used as a starting material. Thus, the process is considerably inconsistent with the characteristic feature of the present invention to use polyamide having a specific end group balance, i.e., an excessive amount of carboxyl end groups, as a starting material.
Japanese Patent Applications Laid-Open Nos. 57-200420(1982) and 58-111829(1983) disclose methods for producing polyamide by melt-polymerizing a diamine component composed mainly of m-xylylenediamine with a dicarboxylic acid component composed mainly of adipic acid. Also, Japanese Patent Application Laid-Open No. 02-245026(1990) discloses a solid phase-polymerized polymer of m-xylylene adipamide. However, there is no description about solid phase-polymerized polymers containing reduced amounts of gels or fish eyes.
As mentioned above, with respect to polyamides comprising a diamine component composed mainly of m-xylylenediamine and a dicarboxylic acid component composed mainly of adipic acid, solid phase-polymerize polyamides which have been successfully reduced in the amounts of gels and fish eyes are not known in the art.